Time Division Multiplexing (TDM), in general, is a method for sending multiple digital signals along a single telecommunications pathway. In general, this is accomplished by deriving two or more apparently simultaneous channels from a given frequency spectrum by interleaving pulses, representing bits, from different channels. In some TDM systems, successive pulses represent bits from successive channels. In other systems different channels take turns using the channels for a group of successive pulse-times (a so-called “time slot”). A primary usage for TDM is the optimal transmission and switching of digitally encoded speech signals within a circuit-switched network, for example, a telecommunications network.
The development of modern optical networks was driven by a specific set of requirements. The first is synchronicity—all clocks in the system must align with a reference clock. Second, traffic must be routed from end exchange to end exchange without any concern for intermediate exchanges, where the bandwidth can be reserved at a fixed level for a fixed period of time. Next, frames of any size must be allowed to be removed or inserted into an existing frame of any size. The network must also be easily manageable, with the capability of transferring management data across the network links, provide a high level of recovery from faults, provide high data rates by multiplexing any size frame, limited only by technology, and minimize the occurrence of bit rate errors. Out of these requirements arose SONET and SDH, which became the two standards used for TDM connections.
SONET is a standard for connecting fiber-optic transmission systems, in particular, optical telecommunications systems, and it used primarily in the United States and Canada. SONET was proposed by Bellcore in the middle 1980s and is now an ANSI standard. SONET defines interface standards at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (about the same as a T-3 line) to 2.48 Gbps. Prior rate standards used by different countries specified rates that were not compatible for multiplexing. With the implementation of SONET, communication carriers throughout the world can interconnect their existing digital carrier and fiber optic systems.
SDH is the international equivalent of SONET and was standardized by the International Telecommunications Union (ITU). SDH is an international standard for synchronous data transmission over fiber optic cables. SDH defines a standard rate of transmission at 155.52 Mbps, which is referred to as STS-3 at the electrical level and STM-1 for SDH. STM-1 is equivalent to SONET's Optical Carrier (OC) levels-3.
Employing these standards in the designing and provisioning TDM connections in a network is one of the major tasks in a telecommunications carrier's daily operations. However, this task is complex and tedious, as it requires engineers to have detailed knowledge not only of TDM connections, but also of the architecture of the specific network elements that are to be connected.
Traditionally, creating these connections involves breaking down the larger network into subnetworks, creating connections within each of the subnetworks, and then combining the subnetwork connections as necessary. The drawbacks to this technique become obvious when creating an end-to-end connection that traverses multiple subnetworks. Since each subnetwork connection is created independently, linking them all together usually requires manually configuring the building of the interconnections, as well as potentially requiring judgment calls on the part of the engineer.
Thus, there exists a need for a technique that serves to unify the process of connection creation from an aggregation of subnetwork creations into a complete and coherent process. This new technique must take into account the special requirements of the subnetworks, while automating and clarifying the connection generation process and removing potentially idiosyncratic manual configuration from the equation.